Over the past several years, numerous materials have been introduced for use in advanced CMOS processes to continue scaling and increase device performance. Among these materials are high-electron-mobility III-V compounds, which can be used as substrates for CMOS technology. When used as substrates for high performance devices, the III-V materials have to be compliant with the strict metal contamination specifications or requirements of a semiconductor manufacturing facility. Meeting these specifications or requirements is a challenge for the cleaning and the etching techniques employed, especially because the surface roughness of the cleaned substrate may not be affected.
LEC (liquid encapsulated Czochralsky) and VGF (vertical gradient freeze) are two of the main growth methods for III-V materials, e.g., GaAs materials. Very often GaAs substrates grown by these methods can show high metal contamination levels on the order of between 1×1018 and 1×1020 at/cm3. These metal contaminations can include metals such as noble metals such as Au, Ag, Pt, and Cu, or can be include common metal contaminants such as Fe, Cr, Ni, and Zn. For example, in GaAs materials, the concentration of Cu contaminant can range from 1×1019 up to 1×1020 at/cm3, especially within the 5-10 microns from the surface down to the bulk of the substrate. This is known as bulk Cu contamination and most likely originates from the polishing slurries used in the fabrication process. After a chemical-mechanical process step of the surface Cu, or in general the metal contaminant, rapidly diffuses deeply into the substrate, even at room temperature.
Other III-V materials besides GaAs, like GaP, GaSb, InAs, InP, InSb and combinations thereof may show similar high contamination levels as described above.
In order to clean the III-V semiconductor substrate or in other words, to remove the metal contaminants from the substrate, a thick layer of contaminated substrate needs to be removed, e.g. by etching, without metal re-plating occurring on the bare, e.g. etched, substrate afterwards. Re-plating is a phenomenon that may occur because noble metals like Cu and Au have high oxidation potentials and are able to oxidize the III-V semiconductor substrate. As a result, re-deposition of these noble metals from the cleaning solutions onto the III-V semiconductor substrate, also known as re-plating, can occur through a redox reaction. This behaviour makes the noble metals very hard to remove with standard chemistries.
Song, et al., J. of Crystal Growth 264, 98-103 (2004), discloses a method of treating GaAs surfaces to reduce Cu contamination. Nevertheless, GaAs substrates treated with their method showed a Cu concentration of about 12×1010 Cu at/cm2, which may yet be too high to be accepted in a semiconductor manufacturing facility. Moreover, their method may not entirely obviate the need for additional polishing followed by cleaning.